This invention relates to detecting mass flow of a fluid in a conduit.
In known techniques for measuring mass flow, temperature sensitive resistance-wire coils are wound at two or more locations along the length of a sensing tube through which the fluid flows. At least one of the coils is heated by an electric current. Some of the resulting heat is exchanged with the fluid flowing in the sensing tube. An upstream coil tends to lose heat to the fluid and thus cools down while a downstream coil gains heat from the fluid and warms up. The resulting temperature differential between the coils is detected as a measure of the mass flow of the fluid.
The coils are typically wound of an iron-nickel alloy having a mildly positive temperature coefficient (PTC); as the temperature of the alloy rises, so does its resistance. Some wire-wound PTC sensors have temperature coefficients in the range of 1-4%/.degree.C. Because the temperature of each coil affects its resistance, the temperature differential may be determined by a bridge circuit in which the coils form two legs. Changes in the temperature of the ambient and changes in the original temperature of the flowing fluid as it enters the sensor affect the differential coil temperature and thus the measurement of fluid flow.
For measuring relatively high mass-flow rates, the fluid flow may be split between a main conduit and a parallel smaller-diameter sensing tube. The coils are wound on the sensing tube instead of the main conduit. The measurement obtained in the split-flow scheme is affected by changes in the ratio of fluid flows in the main conduit and in the sensing tube caused by, e.g., debris in the conduit or tube, or changes in gas viscosity caused by temperature or pressure shifts.
The foregoing techniques are non-invasive in that no part of the sensor equipment is positioned inside the sensor tube. Mass flow may also be measured by invasive sensors which are immersed in the flow gas within the sensing tube. It is known to form such a sensor using positive-temperature-coefficient materials (PTC) such as barium-titanate (a polycrystalline ceramic). It is also known to use a barium titanate PTC thermistor as an air-flow detector based on the shift in dissipation constant that occurs with different ambient conditions.
The resistance-temperature characteristics of a PTC thermistor include a region below a transition temperature (Curie temperature) in which the resistance is relatively constant or declines slowly with temperature rises, and a region above the transition temperature in which the resistance rises sharply with even small temperature increases.
The PTC thermistor may be operated in a constant-temperature mode in which the power supplied to the device is permitted to vary to enable the device always to tend toward the Curie point.